Subpopulations of lymphocytes in tissue and circulation in clinical immune disorders have been characterized according to membrane antigen phenotype. These findings suggest that there are abnormal numbers of helper/inducer or cytotoxic/suppressor cells. However, because present monoclonal antibodies react with both cytotoxic and suppressor cells, it is frequently ambiguous as which cells are relatively deficient or in excess. We have recently developed a mouse monoclonal antibody (anti-Cyt-1) against human T lymphocytes participating in the cytotoxic T cell (CTL) response but not in suppression. It reacts with suspensions enriched with CTL, stains the infiltrating lymphocytes of target tissues with acute graft-versus-host disease (GVHD), and responder cells depleted of Cyt-1+ cells fail to develop CTL in a mixed lymphocyte culture but do develop specific suppressor cells. We propose herein to fully characterize the lymphocyte subpopulation defined by this antibody, membrane phenotype, antigen specificity and mechanism of killing, to isolate and chracterize the associated antigen and relate antigen density to maturation and function, to study the immunopathology of lymphocyte infiltrates in tissues, and finally to better define the subpopulations of T lymphocytes using this antibody. The techniques used in this proposal include cloning of hybridomas, mixed lymphocyte cultures, indirect immunoperoxidase, and fluorescence activated cell sorting. Two long term objectives of these studies are to better define the immune imbalance in diagnostic specimens, and understand the CTL response and its regulation.